1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to a cell search apparatus and method for a UE (User Equipment) in an asynchronous CDMA (Code Division Multiple Access) mobile communication system.
2. Description of the Related Art
In a CDMA mobile communication system, particularly IS-95, an MS (Mobile Station) acquires PN (Pseudo Noise) code timing upon power-on (initial cell search). Then the MS detects multi-path signal components for rake demodulation with the PN code timing maintained (multi-path search), and searches for a neighbor cell to which the MS is to be handed off (neighbor cell search). If a slotted mode is set in an idle state, when the MS wakes up from sleep, it re-acquires PN code timing lost in the sleep state (reacquisition). A searcher takes charge of the initial cell search, multi-path search, neighbor cell search and reacquisition.
The initial cell search, multi-path search, neighbor cell search, and reacquisition will be described in more detail.
The searcher of the MS in the IS-95 CDMA mobile communication system uses a forward pilot channel to acquire synchronization, that is, the PN code timing. The forward pilot channel has only a scrambled PN code while data is unmodulated, as transmitted from a base station (BS) to all MSs within the cell area of the BS to allow the MSs to acquire the PN code timing. Since all BSs in the IS-95 system are synchronized with one another by GPS (Global Positioning System) satellites, they are identified by their own PN offsets.
Therefore, the MS acquires the PN code timing of the BS on the forward pilot channel upon power-on and thus acquires synchronization to the BS. This is the initial cell search. The multi-path search is a process of searching a predetermined area around the PN code timing, a window and thus detecting a variation in the PN code timing and the multi-path components of a received signal.
To determine whether a handoff is required, the MS measures the signal strengths of forward pilot channels from neighbor cells using information about the PN code timings of the neighbor cells received from the BS, which is the neighbor cell search. Finally, if the MS operates in a slotted mode to increase a waiting time in an idle state, the PN code timing lost in a sleep state is reacquired by searching the window when it wakes up from sleep. This is the reacquisition.
The MS performs the initial cell search, multi-path search, neighbor cell search, and reacquisition using the forward pilot channels. The operations are similar. Hence a single searcher takes charge of them. That is, the searcher functions correspondingly for each of the operations.
IS-95 has been evolved to the future generation communication system. A future generation communication system, UMTS (Universal Mobile Telecommunication System) is an asynchronous CDMA system in which Node Bs are not synchronized to one another. In UMTS, a UE performs initial cell search, multi-path search, neighbor cell search, and reacquisition like an MS in IS-95.
Although IS-95 and UMTS commonly adopt CDMA, they differ in synchronization. Therefore, they operate differently for initial cell search, multi-path search, neighbor cell search, and reacquisition.
A cell specific code is assigned to each Node B to identify it in UMTS. Assuming 512 cells exist in UMTS and one Node B is in each cell, the 512 Node Bs are identified by their cell specific codes.
To search for a covering Node B, the UE must search the 512 Node Bs by checking the phases of their cell specific codes. Thus, a long time is taken for the cell search. Accordingly, a multi-step cell search algorithm is employed in UMTS because of inefficiency of a general cell search algorithm for the cell search. To implement the multi-step cell search algorithm, the Node Bs in the UMTS system, for example, the 512 Node Bs are classified into 64 groups Group 0 to Group 63 each having 8 Node Bs and a different group specific code assigned to it. Each of the Node Bs in each group uses a different scrambling code for CPICH (Common Pilot Channel). Therefore, the UE can search for a covering Node B by its scrambling code.
The multi-step cell search includes the following three cell search steps. Cell search step 1: the UE receives a P-SCH (Primary Synchronization Channel) signal from a Node B and acquires the timing of a slot having the highest power. Cell search 2: the UE acquires frame synchronization and detects a Node B group to which it belongs from an S-SCH (Secondary Synchronization Channel) signal received from the Node B using the slot timing information. Cell search 3: the UE performs an exhaustive search for the covering Node B by a CPICH signal from the Node B using the frame synchronization and Node B group information.
The above three-step cell search algorithm has been designed for the initial cell search in UMTS. Aside from the initial cell search, however, a searcher of the UE must perform multi-path search, neighbor cell search, and reacquisition. A searcher configured to operate by the three-step cell search is good at the initial cell search but limited in performing multi-path search, neighbor cell search and reacquisition. The limitations of the searcher employing the three-step cell search structure in multi-path search, neighbor cell search, and reacquisition are as follows.
(1) A scrambling code timing window needs to be searched at a high rate of tens of Hertzes to adaptively cope with a rapid channel environmental change in the multi-path search. However, the three-step cell search is not suitable for such rapid search.
(2) Information about the scrambling code numbers or scrambling code timing of neighbor cells can be provided from a Node B in the neighbor cell search. If the Node B provides no information, the three-step cell search can be employed. On the other hand, if the Node B provides either or both of the scrambling code number information and the scrambling code timing information, another search method may have better performance in the neighbor cell search.
(3) To increase the waiting time of the UE, scrambling code timing must be acquired as fast as possible in the reacquisition. Yet, the three-step cell search method is not suitable for the rapid scrambling code timing acquisition. When an off-line reacquisition is required to further increase the waiting time, cell search 1 and cell search 2 in the three-step cell search method are not available for off-line search.
Besides, a so-called “timing reference” problem may be generated. If the UE is provided with four searchers for cell search 1, cell search 2, cell search 3, and the multi-path search, neighbor cell search, and reacquisition, timing reference is required for search transition with reliable timing transition.